Recently, communication services with large communication capacity, including video service for smart-phones, have been provided. Accordingly, there is a need for greatly increasing the existing communication capacity and a DWDM (Dense Wavelength Division Multiplexing) type communication has been adopted to increase communication capacity using optical fibers that were installed already. The DWDM type communication simultaneously transmits light having several wavelengths through one optical fiber, using a phenomenon that light signals having several wavelengths do not interfere with each other even if they are simultaneously transmitted through one optical fiber, because laser lights having different wavelengths do not interfere with each other.
At present, NG-PON2 (Next Generation-Passive Optical Network version2) is internationally under consideration as a standard and NG-PON2 sets four channel wavelengths for downward optical signals for subscribers from stations. The wavelength spacing of the four channels is set to 100 GHz or 200 GHz.
Under NG-PON2, one subscriber is supposed to select and receive one wavelength, in which it is possible to receive a downward optical signal by inputting a channel optical signal having a specific wavelength to an optical receiver, using a fixing device separating wavelengths. However, an optical line is not dynamically assigned to the optical receiver that separates specific fixed wavelengths to optical fibers and receives light regardless of the type of the wavelength coupled to specific optical fibers, so it is difficult to manage optical lines.
In order to solve this problem, wavelength tunable optical receivers that can dynamically determine a reception wavelength have been developed. As wavelength tunable filter used in the wavelength tunable optical receivers, filters that transmit only specific wavelengths by alternately depositing amorphous silicon and SiO2 on a glass substrate are generally used.
FIG. 1 illustrates a wavelength tunable filter employed in U.S. Pat. No. 6,985,281. The wavelength-tunable filter in the US patent transmits only specific wavelengths by alternately depositing amorphous silicon and SiO2 on a glass substrate used in this art. In this US patent, the amorphous silicon used as a spacer has a change in refraction index according to temperature of about 10−4, so when a wavelength-selective filter is changed in temperature about 1° C., the frequency of light passing through the wavelength-selective filter changes by about 10 GHz. The wavelength tunable filter has only one transmissive peak in a wavelength band of dozens of nanometers, so amorphous silicon used as a spacer in FIG. 1 should be manufactured with a precise thickness to make the filter correspond to predetermined four-wavelength channel. However, it is very difficult to adjust the thickness of amorphous silicon, so when a wavelength tunable filter is usually manufactured with a difference of several nanometers from a desired wavelength, so it is difficult to manufacture a wavelength tunable filter.
Meanwhile, NG-PON2 standard proposes an APD (avalanche photo diode) as a light receiving element that receives an optical signal having a wavelength passing through a wavelength tunable filter. Further, a wavelength tunable optical receiver should receive high-speed signals in 10 Gbps class, so it should be manufactured such that signal distortion is not generated. In order to receive well signals in 10 Gpbs class, RF impedance for a signal transmission line of a package should be matched well so that the signal transmission line receives well signals in 10 Gbps class. The wavelength tunable filter transmits different wavelength, depending on a change in temperature, so it is required to change the temperature of the wavelength tunable filter in order to tune and receive a specific wavelength.
In FIG. 1 illustrating the US patent, it is possible to adjust the temperature of a wavelength tunable filter using a heater formed in a thin film type in the wavelength tunable filter. However, when the heater is used, the temperature can be easily increased, but is manually decreased by dissipating heat, so it is difficult to adjust time to decrease the temperature.
Further, it is required to adjust the temperature of a wavelength tunable filter at least at a temperature higher than the temperature of the external environment in order to change and keep the temperature of the wavelength tunable filter at a predetermined level using a heater. Accordingly, when the temperature of the external environment changes up to 85° C., it is required to adjust the transmissive wavelength of a wavelength tunable filter by changing the temperature of the wavelength tunable filter at least over 85° C., preferably 105° C. NG-PON2 requires tuning of 2.4 nm wavelengths, so a wavelength tunable filter should be operated at least within 105 to 13° C. However, this temperature is very high and causes a problem with temperature stability of a polymeric adhesive such as epoxy, so epoxy cannot be used when a wavelength tunable filter is assembled.
A communication system under NG-PON2 requires an inexpensive optical element and a TO (transistor outline) type package is used as an inexpensive optical element package. The TO type package is composed of a stem in which electrode pins insulated and sealed by glass are inserted in a plurality of through-holes, and a cover that covers optical parts on the stem.